Many offshore oil field developments utilize floating production systems consisting of semi-submersible units, shuttle tanker fleets and sub-sea wells/flowlines, because the oil and gas reserve sizes are not significantly high to favour the use of a massive gravity structure. The majority of these structures are entirely constructed of steel plates, beams and tubulars joined together by welding; they may have to operate in an environment frequented by drifting first-year or multi-year ice, mobile bergybits/growlers and extreme hostile waves. Consequently, they are subjected to considerable deterioration and damage due to corrosion fatigue cracking at the welded joints and impact with the drifting ice field or mobile ice masses. Furthermore, the commercial transport of oil and gas across ice-bound northern regions requires ice-transiting ships capable of breaking through 4-5 m thick ice and larger ice ridges; such ships are therefore subjected to impact stresses.
The recent disasters of Alexander Kielland, Ocean Ranger and Pier Alpha have heightened the need for accurate monitoring of the safety of these offshore structures. Known inspection procedures and techniques to detect and quantify cracks and defects at welded joints of offshore structures include diver assisted Magnetic Particle Inspection (MPI), Alternating Current Field Measurement (ACFM), Eddy Current (EC) and Ultrasonic Techniques. Current acoustical approaches for monitoring fatigue cracks are based on the measurement of sound pressure levels. These are scalar quantities and hence no directional information can be revealed. Further, all of the known techniques have been found to be limited in application because they must operate in the close vicinity of welded joints and become useless if positioned a short distance away from the zone of damage.
Accordingly there is a need for an improved technique for detecting corrosion fatigue and impact fatigue in structures.